This invention relates to a device that protects electronic circuits from damage caused by fast rising voltage transients. The use of electronic devices of all kinds is ever increasing. These devices are often required to share a power source with power hungry machinery. This machinery can and often does inflict large voltage transients onto the power lines. Another source of voltage transients is lightening. These voltage transients can be damaging to any electronic equipment that is not protected against such transients.
Various circuits and devices can be used to provide some protection against damaging voltage transients. These include filtering circuits using inductors and capacitors, varistors, spark gaps, and ferrite beads.
Filtering circuits using inductors and capacitors can absorb the energy contained in a transient, but they have the disadvantage that energy will subsequently be returned to the circuit. Varistors are widely used to protect against voltage transients. However, they have the disadvantage that they fail after absorbing a few transients. There is no way to detect their failure short of removing the varistor and testing it.
Spark gaps will dissipate some of the energy contained in a transient. However, they will not limit the voltage rise to a level that is safe for most electronic circuits. They also cannot react quickly enough to provide protection. Ferrite beads can be used. However, they operate at very high frequencies and can only protect against transients of very short duration.
U.S. Pat. No. 3,480,832 discloses a magnetic variable resistor as a component in an electrical surge arrester. U.S. Pat. No. 3,581,264 focuses more specifically on a magnetic variable resistor. These devices fulfill their claimed objectives. However, their skin effect and the resulting increase in resistance are often less than desirable.
It is therefore a principal object of this invention to provide a magnetic variable resistor that can be easily placed in series with a conductor carrying current to a load, and which will present very little resistance to D.C. currents and low frequency currents, but which will increase substantially in resistance as the rate of change of current increases.
It is a further object of this invention to provide a magnetic variable resistor that will absorb the energy contained in a high voltage transient and dissipate that energy as heat.
A still further object of the invention is to provide a magnetic variable resistor that is capable of operating at frequencies of 10 MHZ or more.
These and other objects will be apparent to those skilled in the art.